Immunocytochemistry suggests that the prevalence of a sub-type of beta-proteins determines the hardness in the epidermis of the hard-shelled turtle.

The corneous layer of the epidermis in hard-shelled turtles largely derives from the accumulation of beta-proteins as indicated by microscopic, in situ hybridization, and immunocytochemical and Western blotting analysis. The expression of mRNAs of one of the most common type of beta-proteins shows higher expression in upper spinosus and pre-corneous keratinocytes of growing scutes. Two beta-proteins of 14-16 kDa, indicated as Tu2 and Tu17 and representing two subtypes of beta-proteins co-accumulate in the thick corneous layer of the epidermis in hard-shelled turtle. The two beta-proteins apparently mix in differentiating and mature corneocytes although Tu2 appears more prevalent than Tu17. The specific role of the different subtypes in the formation of the hard corneous material of the carapace and plastron is not clear. It is hypothesized that the relative amount of beta-proteins belonging to the two subclasses in relation to the alpha-keratin meshwork present in keratinocytes contributes to the formation of a variably resistant and inflexible corneous layer. Tu17 may have a more globular structure than Tu2 and is likely present in denser areas of the corneous layer containing also alpha-keratin. The increase of cysteine-glycine-rich beta-proteins in the matrix located among alpha-keratin filaments may allow the formation of a hard corneous material, probably through increase of cross-bridge formation and hydrophobicity.

Ultrastructural immunocytochemistry for the central region of keratin associated-beta-proteins (beta-keratins) shows the epitope is constantly expressed in reptilian epidermis.

The presence of beta-proteins containing a core-box region in specific regions of reptilian epidermis has been studied by immunological methods. Alpha-keratins are detected by the antibody AK2 that recognizes a sequence toward the C-terminal of acidic alpha-keratins of 48-52kDa. Beta-proteins are recognized by an antibody directed to the core-box region specific for these proteins of 18-37kDa. The AK2 antibody labels with variable intensity alpha-keratin bundles in basal and suprabasal keratinocytes in the epidermis of representative species of reptiles but immunolabeling decreases or disappears in pre-corneous and corneous cells. As opposite, the core-box antibody only labels with variable intensity the dense beta-corneous material formed in pre-corneous and corneous layers of crocodilian and turtle epidermis. In lepidosaurian epidermis the core-box antibody labels the beta-layer while the mesos and alpha-layers are poorly or not labeled. The immunological evidence indicates that beta-proteins are synthesized in the upper spinosus and pre-corneous layers of the epidermis and replace or mask the initial alpha-keratin framework present in keratinocytes as they differentiate into cells of the beta-layer. In the specialized pad lamellae of gecko and anoline lizards charged beta-proteins accumulate in the adhesive setae and may affect the mechanism of adhesion that allows these lizards to walk vertical surfaces. The addition of beta-proteins to the alpha-keratins in upper cell layers of the epidermis recalls the process of cornification of mammalian epidermis where specific keratin-associated proteins (involucrin, loricrin and filaggrin) associate with the keratin framework in terminally differentiating keratinocytes of the stratum corneum.

Forty keratin-associated beta-proteins (beta-keratins) form the hard layers of scales, claws, and adhesive pads in the green anole lizard, Anolis carolinensis.

Using bioinformatic methods we have detected the genes of 40 keratin-associated beta-proteins (KAbetaPs) (beta-keratins) from the first available draft genome sequence of a reptile, the lizard Anolis carolinensis (Broad Institute, Boston). All genes are clustered in a single but not yet identified chromosomal locus, and contain a single intron of variable length. 5'-RACE and RT-PCR analyses using RNA from different epidermal regions show tissue-specific expression of different transcripts. These results were confirmed from the analysis of the A. carolinensis EST libraries (Broad Institute). Most deduced proteins are 12-16 kDa with a pI of 7.5-8.5. Two genes encoding putative proteins of 40 and 45 kDa are also present. Despite variability in amino acid sequences, four main subfamilies can be described. The largest subfamily includes proteins high in glycine, a small subfamily contains proteins high in cysteine, a third large subfamily contains proteins high in cysteine and glycine, and the fourth, smallest subfamily comprises proteins low in cysteine and glycine. An inner region of high amino acid identity is the most constant characteristic of these proteins and maps to a region with two to three close beta-folds in the proteins. This beta-fold region is responsible for the formation of filaments of the corneous material in all types of scales in this species. Phylogenetic analysis shows that A. carolinensis KAbetaPs are more similar to those of other lepidosaurians (snake, lizard, and gecko lizard) than to those of archosaurians (chick and crocodile) and turtles.